This invention relates to a piezoelectric device for use in a ultrasonic motor, a piezoelectric actuator, a piezoelectric sensor, and so forth, and also to a method of producing the piezoelectric device.
Recently, a piezoelectric actuator has drawn increasing attention in the field of mobile information equipment and in chemical and medical fields as a novel motor that can accomplish further scale-down and lightening of a motor and that will replace conventional solenoid type motors. The piezoelectric actuator does not generate electromagnetic noise when it is driven, and is not affected by noise, either. To fabricate appliances having a size of a sub-millimeter class, a very small piezoelectric actuator has been required as the driving source of the appliances. A piezoelectric device has been employed in vibration portions of the ultrasonic motor, the piezoelectric actuator, and so forth. Initially, the construction of the piezoelectric device according to the prior art will be explained.
FIGS. 14 and 15 show the construction of the conventional piezoelectric device. Generally, a piezoelectric body 17 produced by processing a sinter of a bulk material is placed at a predetermined position on a substrate made of a metal or silicon. FIG. 14 shows a piezoelectric device fabricated by bonding a substrate 3 and a piezoelectric body 17 through an adhesive 16. The piezoelectric body is produced by polishing a sinter of a bulk material into a desired size and a desired thickness, or by rapping a green sheet and heat-treating it. Such a sinter of the bulk material and the mold from the green sheet are generally at least about 100 xcexcm thick.
On the other hand, the piezoelectric body can be directly formed on the substrate by sputtering or printing in place of the bonding method using the adhesive. FIG. 15 shows the piezoelectric device fabricated by such a direct formation method. Generally, the piezoelectric body formed by sputtering or a sol-gel method has a thickness ranging from hundreds of angstroms to several microns, and the thickness ranges from 50 to hundreds of microns in the case of the printing method.
In either construction, electrodes are disposed on the piezoelectric body 17, and an AC voltage is applied through them.
FIG. 16 shows a schematic construction of an ultrasonic motor using such a piezoelectric device for its vibration portion. The ultrasonic motor comprises a stator 13 and a moving member 7 (rotor). The stator 13 generally includes a vibrator 6 made of a flexible material and a piezoelectric body 17 equipped with electrodes. The vibrator 6 and the piezoelectric body 17 are bonded by an adhesive 16. When an AC voltage is applied to the piezoelectric body 17 in the ultrasonic motor having such a construction, the force is generated by the piezoelectric effect, propagates through the vibrator 6, and drives the moving body 7 that is kept in pressure contact with the stator 13 (vibrator 6).
As typified by the explanation given above, the basic construction of the piezoelectric device is the bonding construction of the piezoelectric device and the substrate by the adhesive, or the construction in which the piezoelectric body is directly formed on the substrate.
However, these conventional constructions are not free from the following problems. When the adhesive is used, the force generated from the piezoelectric body is reflected irregularly or absorbed by the adhesive layer during its propagation, with the result being not only the drop of electrical and mechanical performance and reliability of the vibration portion but also peeling on the bonding interface with the piezoelectric body. This problem becomes particularly serious when the vibration portion of the piezoelectric device is used as a driving source of a miniature structure because its size is of a sub-millimeter class and the influences of the adhesive layer become relatively greater.
The essential cause of such a problem lies in that the adhesive layer and its boundary surface are dynamically unstable. To solve this problem, methods of directly forming the piezoelectric body on the vibrator have been examined vigorously in recent years.
Representative methods of producing directly the piezoelectric body include sputtering and CVD. Though they have the merit that devices having a very small size can be formed, they are not yet free from the problem that an extremely large number of fabrication steps are necessary. To create driving force of the actuator, the piezoelectric layer must be formed to a thickness of up to dozens of microns, but these methods cannot easily give such a thickness.
Another typical method is a screen printing method that has been used widely for a piezoelectric element of ink jet printer heads. This production method applies a piezoelectric paste to a substrate, followed by drying and firing. However, since firing is conducted at a temperature of 1,000xc2x0 C. or above, there occur the problems that heat cracks develop due to the difference of the thermal stress between the substrate and the piezoelectric paste and that a heat-resistant substrate must therefore be used.
A hydrothermal method is known as still another method of forming directly the piezoelectric body. This method allows a strongly alkaline solution of a ferroelectric ceramic material consisting of lead zirco-titanate (hereinafter called xe2x80x9cPZTxe2x80x9d) to react in an autoclave, and forms PZT on a titanium or titanium oxide substrate. According to this method, the substrate capable of forming PZT is limited to titanium or a titanium-containing material.
It is therefore an object of the present invention to provide a method of producing a stable piezoelectric device having improved properties, wherein the piezoelectric device, can solve the problems described above and can simplify the production process.
To accomplish the object described above, the present invention forms a ultra-fine particle layer between a substrate and piezoelectric layer in a construction of a piezoelectric device, wherein the ultra-fine particle layer consists of substantially the same main component as the main component of the piezoelectric layer.
In the present invention, the ultra-fine particle layer is formed between the substrate and piezoelectric layer, and this ultra-fine particle layer has substantially the same crystal structure as the crystal structure of the piezoelectric layer.
The ultra-fine particle layer and the piezoelectric layer contain titanium, zirconium and lead. Furthermore, the particle diameter of ultra-fine particles forming the ultra-fine particle layer is not greater than 1 xcexcm.
To produce the piezoelectric device having the construction described above, a production method according to the present invention comprises the first step of forming an ultra-fine particle layer on a substrate and the second step of forming a piezoelectric layer on the ultra-fine particle layer.
The first step of forming the ultra-fine particle layer forms the ultra-fine particle layer by applying a paste-like solution containing the ultra-fine particles to the substrate.
The first step of forming the ultra-fine particle layer forms the ultra-fine particle layer by ejecting and depositing the ultra-fine particles onto the substrate.
Alternatively, a production method of the present invention comprises the first step of forming an ultra-fine particle layer on a substrate and the second step of bonding a piezoelectric body onto the ultra-fine particle layer.
In the second step of bonding the piezoelectric body onto the ultra-fine particle layer, the hydrogen bonds are used as means for bonding the ultra-fine particle layer and the piezoelectric layer.